The feedwater supply system for many conventional boiling water reactors and especially for simplified boiling water reactors (collectively referred to as BWRs) is a conventional yet simplified system, characterized by two stages of pumping. These pumping stages raise the feedwater from the below-atmospheric pressures at the source of the feedwater flow--namely, the condenser hotwell--to the pressures needed for injection into the BWR feedwater spargers positioned inside the reactor. The first or lowermost pumping stage, customarily termed the "condensate" stage, contains oondensate pumps having pump discharge pressures, at design flow, of approximately 500 psig, with shutoff heads of approximately 600 psig. The second or uppermost pumping stage, customarily termed the "feedwater" stage, contains feedwater pumps capable of increasing the feedwater supply pressure to approximately 1250 psig.
The pumping burden, at both stages, is commonly shared by redundant pumps. For example, a configuration featuring three condensate pumps (and also three feedwater pumps) each having 50% rated flow capacity is one attractive configuration. One pump from each stage may be held in a standby mode, to be brought into service in case an operational pump requires shutdown for any reason.
During normal operation, these pump units are motor-driven by power from the main station power supply, with the feedwater pumps having adjustable-speed drives to provide feedwater regulation to the reactor. However, during some loss-of-coolant accidents (LOCAs), the reactor must be supplied with additional coolant and cooled--that is, the reactor coolant level must be maintained high enough to cover all of the reactor nuclear fuel assemblies. Such additional coolant must be supplied by reliable emergency cooling systems which draw power from reliable alternative sources.
Loss-of-coolant inventory conditions may occur because of a pipe break (i.e., a LOCA), loss of feedwater supply, or because a safety-relief valve has stuck-open and failed to reclose following a transient. Coolant must be maintained, or must be rapidly replenished following its loss during such accident conditions, to keep the reactor core supplied with coolant to counteract core decay heat generation. These systems which must function to prevent exceedence of core temperature limits comprise the "emergency core cooling network" (ECCS). Core decay heat generation results from the radioactive decay of fission products and continues even after the fission itself has stopped.
In addition, coolant inventory is depleted within the reactor through processes of boiling and evaporation as the hot reactor coolant continues to receive decay heat from the core. As a result, an intermittent or even continuous replenishment of coolant is needed in the long term. The replenishment rates may be large immediately following an accident. Thereafter however, replenishment rates diminish as time goes on and the decay heat generation rate decreases. Exoept for very small LOCAs, replenishment of coolant must continue until the break can be isolated and normal coolant inventory level reestablished inside the reactor. For certain accidents, replenishment must continue until the region of the containment immediately outside the reactor pressure vessel can be flooded to an elevation above the top of t he core active fuel level or the break, whichever is higher.
Several emergency core cooling systems with independent power supplies have evolved for responding to a LOCA for nuclear power reactors in general and for boiling water reactors in particular.
Conventional BWR ECCS networks, for example the BWR/3 through the BWR/6 model BWR designs by GE Nuclear Energy, utilize a combination of pumping systems and power supplies to pump coolant into the reactor following any loss-of-coolant inventory condition.
Water is typically used as the emergency coolant for BWRs. The source of water can be any available quantity of water within the power station or its premises. For example, the BWR/3 through BWR/6 reactors typically draw emergency coolant from a containment suppression pool. This suppression pool provides water which is assured, is available in large amounts, and is generally of a quality that is not particularly harmful to the reactor vessel or the nuclear steam supply system piping or equipment.
Because the containment suppression pool is conventionally located low in the containment relative to the higher-elevation nuclear reactor, a break in certain pipes connecting to the reactor can allow injected coolant to be drained back out of the vessel. Such BWR designs result in extremely long pump-operating requirements for the pumping systems that provide the necessary emergency coolant inventory replenishment action.
Thus, the conventional BWR designs have several drawbacks relating to emergency core cooling resulting from the extremely long pump duty cycles needed to meet coolant replenishment requirements. For example, both the pumps and the piping networks as well as the power supplies that power the ECCS have heretofore been costly dedicated systems having high reliability ratings. Such high reliability ECCS design often is achievable only by providing redundant components or even redundant pumping loops. Such redundancy in systems results in significant cost increases for the power station.
It is possible to use the main system generator as a source of power for ECCS pumps during some LOCAs. However, in some important accident scenarios, electrical power from the main generator is hypothesized to be unavailable. For example, the main generator itself may be in a shorted condition (e.g., shorted windings), or the main generator may otherwise have been taken offline during the LOCA.
For conventional BWRs, safety-grade diesel generators are installed that supply the necessary reliable ECCS network electrical power. These diesel generators are used where in-house electrical power has been interrupted from the generally two independent offsite grid power supplies into the station, as well as from the power station main turbine-generator. (The power station's in-house ("hotel") load can be furnished by the station main turbine-generator, but only if the reactor steam source has not become isolated.)
A loss of power from these preferred sources would result in the automatic start-up of the diesel generators, and the subsequent progressive loading onto their emergency buses of the motor loads for ECCS pumps and other emergency equipment. For conventional systems, such diesel generators must be rated to operate continuously for as long as 90 days and typically must have an 8-day supply of fuel on hand.
Advanced simplified types of BWRs--termed SBWRs--position the suppression pool previously discussed at a high elevation in the containment vessel relative to the core top-of-active-fuel (TAF) elevation. This elevation of the suppression pool overcomes the long-term need for continuous pumped coolant injection into the reactor. The suppression pool is connected via a plurality of pipes directly to the reactor, with valves--typically check valves--that prevent the discharge of high-pressure reactor coolant into the suppression pool during routine reactor power generation. This system of pipes and valves is termed a "gravity-driven cooling system (GDCS)", and along with associated venting systems, represents the entire ECCS network for certain SBWRs.
If a loss-of-coolant inventory condition occurs, as detected by reactor water level measurements, the SBWR reactor is promptly depressurized to the suppression pool pressure level using a venting system. When the reactor pressure has fallen to a low pressure level (such as 30 psig), the hydrostatic head created by the elevated suppression pool initiates flow of suppression pool water into the reactor. The suppression pool includes sufficient water such that during a LOCA, both the reactor as well as the region of the containment external to the reactor (the "drywell") can be flooded to a level moderately higher than the TAF level.
The maintenance of adequate reactor coolant inventory in these SBWRs thus no longer depends at any time on coolant inventory replenishment (pumping) by ECCS pumps. The flooding of the reactor and/or drywell by the GDCS using suppression pool water keeps the reactor core inundated. Any boiloff of evaporated coolant passes to the suppression pool through latched-open depressurization valves, and returns to the core by gravity refill via GDCS pipelines.
A design goal for SBWR is not only to avoid exceeding core temperature limits during the course of any design basis accident, but also to provide ample margin against such occurrence. This assured margin is attained by specifying no core uncovery condition shall occur, even briefly, during such accidents. However, any added systems that provide this margin are not required to meet safety-grade design criteria, and these systems are taken as backups to, but not part of the ECCS network itself.
The advantage to added or backup systems that are not required to be part of the ECCS network is that they can be designed to less-stringent criteria, which translates to less expense. At the same time these added or backup systems provide important enhanced investment protection to the power station because they further reduce the risk of core damage given an accident.
To insure adequate coolant inventory (margin) in the short term while the reactor is undergoing depressurization--before the initiation of GDCS flow--the SBWR reactor vessel is designed to contain excess water, relative to conventional BWRs. This extra water is contained in a zone starting with the TAF and extending up to the water level at which reactor depressurization signals are initiated (termed "Level-1"). Thus, those SBWRs which use gravity driven cooling can undergo depressurization--which entails a reduction of steam/water inventory from inside the reactor--and still maintain a sufficient vessel residual coolant inventory. The coolant inventory maintains adequate coverage of the core as the reactor is depressurized to low pressure levels.
The zone between TAF and Level-1 in such SBWR reactor designs contains an amount of water corresponding to approximately one minute of rated feedwater flow injection. This amount is substantially larger than in conventional BWR designs which rely on long term ECCS pumped water injection into the reactor during and following reactor depressurization.
Unfortunately, this excess volume leads to a taller reactor vessel, which in turn leads to a larger drywell and larger suppression pool, and thus greater costs for both the reactor vessel and containment.